Stoux City Academy of Science and Letters. 151 
western Dixon County about 10 miles northwest of where 
the Greenhorn beds pass below the Missouri, and overlie 
the thick shale member (Carlile) that overlies the Green- 
horn is sufficient stratigraphic evidence for differentiat- 
ing the two limestone formations. While both are char- 
acterized by the presence of Jnoceramus labiatus, the Nio- 
brara beds contain in addition great numbers of Ostrea 
congesta which are found in many instances attached to 
the Inoceramus shells. Fossils collected by the writer 
were submitted to Dr. T. W. Stanton, of the U. 8. Geo- 
logical Survey, who pronounced them from the Green- 
horn limestone horizon. The thickness of the Greenhorn 
member including the shaly and chalky beds is from 15 
o 25 feet. 
Mr. N. H. Darton, of the U. S. Geological Survey 
was the first to recognize the relations of these lime- 
stones. The facts became apparent to him through a 
study of well records from southeastern South Dakota. 
Since the distinction between the Niobrara and 
Greenhorn limestones has become understood it is evi- 
dent that a slight revision is necessary in the geology of 
Woodbury and Plymouth counties as mapped by the 
Iowa geological survey and in the standard section of the 
Cretaceous along Big Sioux River. In the south part of 
this area the Goethe has been called Niobrara. 
Quaternar y deposits—Since the loess has been dis- 
cussed to a certain extent in connection with the topo- 
graphy and will be taken up again together with the 
glacial deposits in the section on geologic history, it is 
not thought necessary to devote much space here to these 
two formations. Over the eroded and uneven surface of 
the Cretaceous is spread this mantle. In many places the 
glacial till is wholly absent; in others there is but a few 
feet of it, and it has not been observed to be above 20 
feet thick. The till is the result of the earlier or Kansan 
ice sheet and of the streams and drifting ice floes from 
the Wisconsin ice sheet which lay to the north of this 
region. The drift consists of irregular masses of bowlder 
clay and beds of sand and gravel and a few scattered 
bowlders. At a few exposures along the escarpment in 
the north part of the county may be seen the glacial 
material, and it has been found in well drilling, especially 
in stream valleys. 
